Integrated circuit with a recessed conductive layer

ABSTRACT

An improved method for making an integrated circuit. That method includes forming a first dielectric layer on a substrate, etching a trench into that layer, then filling the trench with a conductive material. The conductive material is then electropolished to form a recessed conductive layer within the first dielectric layer.

This is a Divisional Application of Ser. No.: 09/422,841 filed Oct. 21,1999, now U.S. Pat. No. 6,383,917.

FIELD OF THE INVENTION

The present invention relates to a method for making integratedcircuits.

BACKGROUND OF THE INVENTION

Integrated circuits are made by forming on a substrate, such as asilicon wafer, layers of conductive material that are separated bylayers of dielectric material. Trenches may be etched into thedielectric layers, e.g., when forming single or dual damasceneinterconnect structures, then filled with a conductive material to formconductive layers.

In the conventional process, generally, the surface of a conductivelayer formed in a trench is substantially flush with the surface of thedielectric layer or layers that insulate the conductive layer. In somecircumstances, such an attribute may be undesirable. Take, for example,an integrated circuit that includes a dielectric layer made from a lowdielectric constant material. Such a dielectric layer may ensure that aconductive layer's RC related delays are reduced. Such a layer, however,may have poor mechanical integrity. To improve the mechanical strengthof the overall dielectric layer, a second dielectric layer, having ahigher dielectric constant and superior mechanical strength, may beformed on the layer with the low dielectric constant. Because thatsecond layer has a relatively high dielectric constant, however, some ofthe RC delay reducing benefit, which the low dielectric constantmaterial provides, will be lost when such a layer is used to form partof the overall dielectric layer.

There is thus a need for an improved method for making an integratedcircuit in which the surface of a conductive layer formed in a trench isrecessed from the surface of the dielectric layer or layers thatinsulate the conductive layer. There is also a need for such a methodthat produces an integrated circuit that has acceptable RCcharacteristics, while using mechanically strong dielectric layers toseparate the conductive layers. This application describes such amethod.

SUMMARY OF THE INVENTION

An improved method for making an integrated circuit is described. Thatmethod comprises forming a dielectric layer on a substrate, then etchinga trench into that layer. After filling the trench with a conductivematerial, the conductive material is electropolished to form a recessedconductive layer within the dielectric layer. In a preferred embodiment,a dielectric layer having a relatively high dielectric constant isformed on top of a dielectric layer having a relatively low dielectricconstant. A trench is etched through the upper layer and into the lowerlayer, then filled with a conductive material. That conductive materialis then electropolished to form a recessed conductive layer that isseparated from the dielectric layer that has a relatively highdielectric constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 g represent cross-sections of structures that may result whencertain steps are used to carry out an embodiment of the method of thepresent invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

An improved method for making an integrated circuit is described. Inthat method, first dielectric layer 101 is formed on substrate 100, asshown in FIG. 1a. Substrate 100 may be any surface, generated whenmaking an integrated circuit, upon which a dielectric layer may beformed. Substrate 100 thus may include, for example, active and passivedevices that are formed on a silicon wafer such as transistors,capacitors, resistors, diffused junctions, gate electrodes, localinterconnects, etc. . . . Substrate 100 also may include previouslyformed dielectric layers that separate such active and passive devicesfrom the conductive layer or layers that are formed on top of them, orthat separate various conductive layers from each other.

First dielectric layer 101 may comprise any material that may insulateone conductive layer from another. Examples of such materials includesilicon dioxide (either undoped or doped with phosphorus (PSG) or boronand phosphorus (BPSG)); silicon nitride; silicon oxy-nitride; porousoxide; an organic containing silicon oxide; or a polymer. Preferred arepolymers with a low dielectric constant: preferably less than about 3.5and more preferably between about 1.5 and about 3.0. When layer 101 ismade from a polymer having a low dielectric constant, the capacitancebetween various conductive elements that are separated by layer 101should be reduced, when compared to the capacitance resulting from useof other conventionally used dielectric materials—such as silicondioxide. Such reduced capacitance may decrease the RC delay that wouldotherwise exist and may also decrease undesirable cross-talk betweenconductive lines.

First dielectric layer 101 may comprise an organic polymer. Such organicpolymers include, for example, polyimides, parylenes, polyarylethers,organo-silicones, polynaphthalenes, and polyquinolines, or copolymersthereof. A commercially available polymer sold by Allied Signal, Inc.,under the trade name FLARE™, may be used to form first dielectric layer101. When first dielectric layer 101 comprises a polymer, it ispreferably formed by spin coating the polymer onto the surface ofsubstrate 100, using conventional equipment and process steps.

First dielectric layer 101 may alternatively be made from a compoundhaving the molecular structure Si_(x)O_(y)R_(z), in which R is selectedfrom the group consisting of hydrogen, carbon, an aliphatic hydrocarbonand an aromatic hydrocarbon. When “R” is an alkyl or aryl group, theresulting composition is often referred to as carbon-doped oxide. Whenfirst dielectric layer 101 comprises a carbon-doped oxide, dielectriclayer 101 preferably includes between about 5 and about 50 atom %carbon. More preferably, such a compound includes about 15 atom %carbon.

Examples of other types of materials that may be used to form dielectriclayer 101 include aerogel, xerogel, and spin-on-glass (“SOG”). Inaddition, dielectric layer 101 may comprise either hydrogensilsesquioxane (“HSQ”), methyl silsesquioxane (“MSQ”), or othermaterials having the molecular structure specified above, which may becoated onto the surface of a semiconductor wafer using a conventionalspin coating process. Although spin coating may be a preferred way toform layer 101 for some materials, for others chemical vapor deposition(e.g., plasma enhanced chemical vapor deposition—“PECVD”), a Sol-Gelprocess, or foaming techniques may be preferred. First dielectric layer101 preferably has a thickness of between about 100 and about 1,000nanometers.

In the embodiment of the present invention described with reference toFIGS. 1a-1 g, second dielectric layer 102 is formed on the surface offirst dielectric layer 101. Second dielectric layer 102 preferablycomprises a material that is mechanically stronger than the materialused to form first dielectric layer 101. Some examples of materials thatmay be used to form layer 102 include silicon nitride, silicon dioxide,and silicon oxy-nitride. Second dielectric layer 102 may be formed usinga conventional chemical vapor deposition process, and preferably isbetween about 10 and about 200 nanometers thick. Combining first layer101 (which may enable improved RC characteristics, but has poormechanical integrity) with second layer 102 (which may increase RCdelay, but provide superior mechanical strength) creates an overalldielectric layer that may ensure improved RC performance while retainingsuitable mechanical strength properties.

After forming second dielectric layer 102 on first dielectric layer 101,a photoresist layer is deposited and patterned, e.g., by usingconventional photolithographic techniques, to define a trench that willbe etched into dielectric layer 103—dielectric layer 103 comprising thecombination of layers 101 and 102. After that trench is etched, e.g., byusing a conventional etch process, barrier layer 104 is formed, whichlines the trench bottom and walls. Conductive layer 105 is then formedon barrier layer 104, generating the structure shown in FIG. 1b.

Barrier layer 104 is formed to block diffusion into dielectric layer 103of copper or other elements that may be included in conductive layer105. Barrier layer 104 preferably comprises a refractory material, suchas tantalum, tantalum nitride or titanium nitride, but may be made fromother materials that can inhibit diffusion from conductive layer 105into dielectric layer 103. Barrier layer 104 preferably is between about10 and 50 nanometers thick, and preferably is formed using a conformalchemical vapor deposition process.

Conductive layer 105 may be made from materials conventionally used toform conductive layers for integrated circuits. For example, conductivelayer 105 may be made from copper, a copper alloy, aluminum or analuminum alloy, such as an aluminum/copper alloy. Alternatively,conductive layer 105 may be made from doped polysilicon or a silicide,e.g., a silicide comprising tungsten, titanium, nickel or cobalt.Preferably, conductive layer 105 consists essentially of copper.

Although a few examples of the types of materials that may formconductive layer 105 have been identified here, that layer may be formedfrom various other materials that can serve to conduct electricitywithin an integrated circuit. Although copper is preferred, the use ofany other conducting material, which may be used to make an integratedcircuit, falls within the spirit and scope of the present invention.

Conductive layer 105 may be formed by a chemical vapor or physicaldeposition process, as is well known to those skilled in the art.Alternatively, where copper forms conductive layer 105, a conventionalcopper electroplating process may be used. Such a process typicallycomprises depositing a barrier layer (e.g., barrier layer 104 shown inFIG. 1b) followed by depositing a seed material, then performing acopper electroplating process to produce the copper line. Such a processis described in copending applications Ser. Nos. 163,847 and 223,472(filed Sep. 30, 1998 and Dec. 30, 1998, respectively, and each assignedto this application's assignee), and is well known to those skilled inthe art. Suitable seed materials for the deposition of copper includecopper and nickel.

In a typical process, after conductive layer 105 is deposited, it ispolished, e.g., by applying a conventional chemical mechanical polishing(“CMP”) step, until its surface is substantially flush with the surfaceof dielectric layer 103. Applicant has found, however, that removingsubstantially more of conductive layer 105, without similarly removingadditional amounts of layer 103, to form a recessed conductive layerwithin layer 103 enables processing options that are not available, whenthose layers' surfaces are maintained at substantially the same level.For example, as described below, such a process enables production of anintegrated circuit that maintains the strength characteristics impartedby second layer 102, without that layer adversely affecting the improvedRC properties that first layer 101 ensures.

One way to remove portions of conductive layer 105, without removingsubstantial parts of dielectric layer 103, is to electropolish layer105. When layer 105 comprises copper, an electropolish process may be aparticularly preferred way to generate a recessed conductive layer—ascopper may be difficult to etch or remove in a controlled manner usingother procedures. Such a process will generally require that electricalcontact be maintained to conductive layer 105. In the embodimentdescribed herein, electrical contact may be maintained with conductivelayer 105 because layer 105 is formed on a dissimilar, relatively thin,conductive layer that lines the trench—i.e., barrier layer 104.

The electropolish process itself is well known to those skilled in theart, consisting essentially of contacting the surface to be polishedwith an appropriate solution chemistry, then applying an electricalpotential to that surface. See, e.g., R. Contolini, A. Bernhardt, and S.Mayer, Electrochemical Planarization for Multilevel Metallization, J.Electrochem. Soc., Vol. 141, No. 9, pp. 2503-2510 (September 1994). Sucha process may enable the controlled, selective removal of copper fromthe surface of a copper layer. In one example, portions of a copperlayer may be removed by exposing that layer's surface to a phosphoricacid containing solution, then applying an electrical potential ofbetween about 1 and about 1.5V (with respect to a copper referenceelectrode) for a period of time sufficient to remove the desired amountof copper from the copper layer. The electrical potential may be appliedin a steady state fashion, or alternatively, in a dynamic fashion—e.g.,by using pulsed plating. Preferably, current density is maintainedbetween about 15 and about 20 mA/cm². In a preferred embodiment,conductive layer 105 is polished until its surface 114 is separated fromsecond dielectric layer 102 by at least about 10 nanometers, and morepreferably by at least about 50 nanometers.

As the electropolishing process penetrates and removes copper from theunderlying conductive layer, local electric fields may varysignificantly—especially when the copper layer is polished down to theportions of barrier layer 104, which cover second dielectric layer 102.As this occurs, it may be desirable to optimize the process to accountfor these changes in local fields, which could affect the uniformity ofthe etch rate. This effect may be addressed, for example, by: (1)applying dynamic fields; (2) using certain chemical additives that actas plating suppressors or antisuppressors to modulate theelectropolishing; and/or (3) increasing the surface area of theconductive material—e.g., by adding dummy metal regions to thestructure.

Electropolishing conductive layer 105 produces the structure shown inFIG. 1c. Although the embodiment described above specifies copper forconductive layer 105, preferably formed on a tantalum barrier layer, anelectropolish process may be used for the controlled removal of othermetals, which have been deposited on an underlying, dissimilar,conductive layer.

After electropolishing conductive layer 105, a two step process followsto form second barrier layer 106 to completely encapsulate conductivelayer 105. First, layer 107 (like layer 104 preferably comprisingtantalum, but which alternatively may comprise tantalum nitride ortitanium nitride) is deposited over first barrier layer 104 andconductive layer 105 using, for example, a conventional chemical vapordeposition process. In a preferred embodiment, layer 107 is deposited inan anisotropic fashion such that portions 110, 111 of layer 107(covering conductive layer 105 and horizontal portion 113 of barrierlayer 104, respectively) are substantially thicker than portions 112 oflayer 107 that line the walls of the trench. The resulting structure isshown in FIG. 1d.

Next, portions 112 of layer 107 and portions 115 of layer 104, whichline the trench walls, are removed. In a preferred embodiment, aconventional isotropic etch step is applied to remove substantially allof portions 112 and 115 of layers 107 and 104 from the trench walls.Such an isotropic etch step should remove only part of portions 110, 111of layer 107, which sit on top of conductive layer 105 and on top ofbarrier layer 104. Following that etch step, a conventional CMP step maybe used to remove the remainder of portion 111 of layer 107 andunderlying portion 113 of layer 104, while retaining portion 110 oflayer 107. After such a CMP step, the resulting structure includessecond barrier layer 106 formed on surface 114 of conductive layer 105,as shown in FIG. 1e.

Second barrier layer 106, like layer 104, will serve to prevent anunacceptable amount of copper, or other metal, from diffusing fromconductive layer 105 into any overlying dielectric layer. Althoughsecond barrier layer 106 preferably is made from tantalum, that layermay be made from other materials that can serve this function, as iswell known to those skilled in the art.

After forming second barrier layer 106, third dielectric layer 109 maybe deposited on its surface, as shown in FIG. 1f. Layer 109, like layer101, preferably has a low dielectric constant. The same materials,process steps and equipment used to form layer 101 may be used to formlayer 109. Alternatively, different materials may be used to form thosetwo layers. In the resulting structure, low dielectric constant layers101 and 109 completely surround conductive layer 105, ensuringacceptable RC delay. High dielectric constant layer 102, whichpreferably is mechanically stronger than layers 101 and 109, isseparated from layer 105 to reduce layer 102's effect on conductivelayer 105's RC properties.

After forming layer 109, via 108 may be etched through it, as shown inFIG. 1g. Via 108 will be filled with a conductive material that willcontact conductive layer 105. If layer 109 comprises a polymer, it maybe desirable to form a hard masking layer on top of layer 109 prior toetching the via through it. Such a hard mask may ensure that processsteps used to remove the photoresist or clean the via do not erode layer109. Preferred materials for making such a hard masking layer aresilicon nitride and silicon dioxide, although other materials, such asSiOF, may be used. When a hard mask is formed on top of third dielectriclayer 109, a two step etch process may be required—the first step foretching through the hard mask and the second step for etching throughthird dielectric layer 109.

When copper is used for conductive layer 105, it may be desirable toremove the portion of second barrier layer 106 that separates via 108from conductive layer 105 before filling via 108. Such a removal stepmay help ensure good contact between conductive layer 105 and the filledvia. The conductive material that fills via 108 preferably comprisescopper, but may instead comprise other materials. In this regard,although preferably the same type of conductive material formsconductive layer 105 and fills via 108, different materials may be usedwithout departing from the spirit and scope of the present invention.(Hash marks shown in FIG. 1g indicate that via 108 may be formed withinthe integrated circuit at a distance removed from the surface of thedepicted cross-section.)

After via 108 is filled, the resulting damascene structure includes arecessed conductive layer 105 spaced sufficiently far from dielectriclayer 102 to ensure that layer 102 will not significantly degradeconductive layer 105's favorable RC properties, which low dielectricconstant layers 101 and 109 facilitate. To achieve such a structure, thetrench etched through dielectric layer 103 (shown in FIG. 1b) will bedeeper than it would otherwise have been, if the surfaces of conductivelayer 105 and layer 102 were maintained at substantially the same level.Note, however, that adding the conductive layer recess step does notrequire changing the aspect ratios of via 108 and conductive layer 105from what they would have been had conductive layer 105 not been sunkbelow the high dielectric constant layer. Those aspect ratios may remainsubstantially the same because the reduced thickness of layer 109, whereit lies above layer 102, compensates for the increase in thickness oflayer 101, required for the deeper trench.

As shown in FIG. 1g, the integrated circuit made by this embodiment ofthe method of the present invention includes the following features.Conductive layer 105, preferably comprising copper, is formed onsubstrate 100. Conductive layer 105 is enclosed by barrier layers 104,106, which, in turn, are enclosed by first dielectric layer 101 andthird dielectric layer 109—each preferably having a dielectric constantthat is less than about 3.5. Second dielectric layer 102, preferablymechanically stronger than layers 101 and 109, is formed on layer 101.Because layer 102 may have a dielectric constant that is greater thanthose of layers 101 and 109, layer 102 preferably is separated fromconductive layer 105 by at least about 10 nanometers—and more preferablyat least about 50 nanometers. Via 108 is formed through third dielectriclayer 109 to enable contact between an upper conductive layer (notshown) and conductive layer 105.

Although via 108 is shown in substantial perfect alignment withconductive layer 105, in some processes via 108 may be formed off centerfrom conductive layer 105. If seriously misaligned, part of via 108 mayhave to be cut through layers 102 and 101 in addition to layer 109. (Thesame circumstance may exist if a process is designed so that at leastpart of via 108 must be etched through all three dielectric layers.) Ifthose layers each have different etch characteristics, then etching sucha misaligned via may be more complicated for the process of the presentinvention, than it would have been for a process in which conductivelayer 105's surface is flush with layer 102's surface.

Forming a recessed conductive layer within a trench enables new ways toorient conductive layers with respect to the dielectric layers thatseparate them. As shown above, the method of the present invention canbe advantageously applied to processes for making integrated circuitsthat include a dielectric layer, which comprises both a high dielectricconstant layer and a low dielectric constant layer. When making such adevice, if the high dielectric constant material is positionedrelatively close to the conductive layer, then RC characteristics may bedegraded—when compared to those present when the conductive layer ispositioned relatively close to the low dielectric constant layer only.By forming a recessed conductive layer that is spaced from the highdielectric constant layer, favorable RC properties may be maintainedwhile still retaining the superior mechanical strength benefits that thehigh dielectric constant layer provides.

Unlike prior processes, such a method, which forms a recessed conductivelayer separated from a high dielectric constant layer, enables a processthat does not require sacrificing superior RC characteristics in orderto enhance mechanical strength properties. Such a method enables theresulting device to have the best of both worlds—improved RC propertiesand adequate mechanical strength.

Features shown in the above referenced drawings are not intended to bedrawn to scale, nor are they intended to be shown in precise positionalrelationship. Additional steps that may be included in the abovedescribed method have been omitted as they are not useful to describeaspects of the present invention.

Although the foregoing description has specified certain steps,materials, and equipment that may be used in such a method to make suchan integrated circuit, those skilled in the art will appreciate thatmany modifications and substitutions may be made. For example, althoughthe improved method of the present invention has been described in thecontext of forming a recessed conductive layer to enable separation ofthat layer from a high dielectric constant layer to minimize thatlayer's effect on the conductive layer's RC properties, this inventionis not limited to that particular application. Any process that forms arecessed conductive layer within a trench formed in a dielectric layerusing electropolishing to etch the layer falls within the spirit andscope of the present invention.

Moreover, although the embodiment described with reference to FIGS. 1a-gshows only one overall dielectric layer and one conductive layer, thenumber of conductive and dielectric layers included in the resultingintegrated circuit may vary, as is well known to those skilled in theart. In this regard, the process described above may be repeated to formadditional conductive and insulating layers until the desired integratedcircuit is produced. Accordingly, it is intended that all suchmodifications, alterations, substitutions and additions be considered tofall within the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. An integrated circuit comprising: a conductivelayer comprising copper formed on a substrate; a first conductivebarrier layer formed between the conductive layer and a first dielectriclayer, the first dielectric layer having a dielectric constant that isless than about 3.5; a second dielectric layer formed on the firstdielectric layer but not over the conductive layer, the seconddielectric layer having a dielectric constant that is greater that thedielectric constant of the first dielectric layer and having amechanical strength that is greater than the mechanical strength of thefirst dielectric layer; a second barrier layer formed between theconductive layer and a third dielectric layer, the third dielectriclayer formed on top of the conductive layer and having a dielectricconstant that is less than about 3.5; and wherein the conductive layeris separated from the second dielectric layer by at least about 10nanometers.
 2. The integrated circuit of claim 1 wherein the first andsecond barrier layers are each formed from a refractory metal selectedfrom the group consisting of tantalum, tantalum nitride, and titaniumnitride; the first and third dielectric layers each comprise a polymerselected from the group consisting of polyimides, parylenes,polyarylethers, organo-silicones, polynaphthalenes, and polyquinolines,or copolymers thereof, and the second dielectric layer comprises aninsulating material selected from the group consisting of siliconnitride, silicon dioxide and silicon oxy-nitride.